Xenoreactive natural antibodies in humans and higher primates are directed predominantly at Galα1-3Gal. These antibodies are thought to initiate hyperacute rejection of porcine organ xenografts. The contribution of anti-Galα1-3Gal antibodies to the xenoreactive natural antibody repertoire and to the initiation of hyperacute rejection was tested in a pig-to-baboon cardiac xenograft model. Anti-Galα1-3Gal antibodies were depleted from baboons by extracorporeal absorption of anti-Galα1-3Gal antibodies from plasma using columns with a matrix bearing Galα1-3Galb1-4GlcNAc. Specific removal of anti-Galα1-3Gal antibodies was achieved prior to transplantation as demonstrated by immunoassay. Porcine hearts were then transplanted into these baboons and the outcome of the transplants was analysed. Immunofluorescence revealed little deposition of baboon antibodies in the grafts. The porcine hearts did not undergo hyperacute rejection even though complement activity was ~ 90% of baseline at the time of transplantation. These findings demonstrate that anti-Galα1-3Gal antibodies constitute a major fraction of xenoreactive natural antibodies in primate blood and that these antibodies contribute significantly to the pathogenesis of hyperacute xenograft rejection.